Detection of short-lived fission products in laser-plasma experiments

The on-going development in laser acceleration of protons and light ions, as well as the production of -rays and strong bursts of neutrons by secondary processes now provide a basis for novel high-flux nuclear physics experiments. The generated proton peak flux within the short laser-accelerated bunches can already today exceed the values achievable at the most advanced conventional accelerators by orders of magnitude. The goal of this project is the demonstration of a measuring method to detect short-lived elements for laser-driven nuclear physics, which provides a basis for a further series of experiments. Therefore, an experiment at the PHELIX facility in Darmstadt (Germany) was planned, performed, and evaluated. By using laser pulses of 500 fs duration with energies up to 200 J, proton pulses in excess of 1012 protons with energies above 15 MeV were achieved. These pulses were used for proton-induced fission of 238U in foil targets. To make use of the nonpareil flux, these targets have to be very close to the laser acceleration source, since the particle density within the bunch is strongly affected by the Coulomb explosion and the velocity differences between ions with a different energy. The main challenge for nuclear detection with high-purity germanium detectors is given by the strong electromagnetic pulse caused by the laser-matter interaction close to the laser acceleration source. This can lead to a noise in the measurement or a detector failure. To protect the detector against this pulse, it is placed at a distance from the PHELIX target chamber and is shielded by a metal encasing and a metal-coated polyamide spun-bond fleece. Due to the distance of the detector, the produced nuclides are transported by a fast gas transport system to a carbon filter, where they are collected, directly in front of the detector. In this first proof-of-principle experiment the carrier gas was varied. The identified nuclides include those that have half-lives down to 39 s. These results demonstrate the capability to produce, extract, and detect short-lived reaction products under the demanding experimental condition imposed by the high-power laser interaction.